This invention relates to rotatably drivable threaded fasteners and drivers therefor as well as devices and methods for their manufacture.
Use of powered tools to drive threaded fasteners at high speed and high torque loads results in high forces applied by the driver to the fastener. Although many threaded fastener drive systems, particularly those with a driver-engageable recess in the fastener head, are designed to have surfaces that are engaged by corresponding surfaces on the driver, such ideal surface-to-surface engagement, at best, is difficult to achieve in practice. Rather than surface-to-surface engagement between the driver and fastener, by which the driving load can be distributed over a broad surface area, driver-recess engagement often is concentrated in small areas or points. That may result from a number of factors such as inconsistencies in the manufacture of the fastener or the driver, as well as difficulties encountered in the field. Field-encountered difficulties may include, for example, misalignment of the driver and fastener or inability to fully seat the driver in the recess because of paint or other debris that may have collected in the recess. Even slight misalignment between the driver and the fastener, or a variation of the fastener or driver from design specifications, can result in substantial reduction in the area of contact between the driver and fastener, in many cases resulting in near point-like contact of several portions of the driver and fastener. Application of high torque under such circumstances necessarily results in concentrated stresses in the materials of the driver and the recess that, in turn, can lead to failure of the material, either by plastic deformation or fracture. Even slight plastic deformation of the engagement surfaces of the recess and driver can adversely affect system performance. If the recess deforms to define ramp-like surfaces inclined from the vertical, the driver may xe2x80x9ccam-outxe2x80x9d of the recess under the influence of the applied load. Such cam-out is undesirable, not only because it results in premature or uncontrollable disengagement of the driver and recess, but also because the suddenly disengaged driver can slip onto and damage the work piece. Additionally, excessive stress in the driver blade while driving the fastener can cause the blade to deform in a manner that reduces the surface area contact with the fastener and effectively shifts the region of contact radially inwardly, thereby reducing the effectiveness of driver-recess engagement and increasing the risk of failure. Even when the sidewalls of the driver and recess wings engage in broad surface-to-surface contact, the point at which the resultant force applied by the driver to the recess sidewall typically is at a center of effort located generally at the center region of the sidewall. Consequently, the resultant force is applied at a location that is substantially radially inwardly from the outermost extremity of the sidewall. The foregoing difficulties may be encountered whether the fastener is being driven in an installation or a removal direction. In many applications, the ability to remove the fastener quickly and effectively is at least, if not more, critical than its installation.
A number of recess and driver engagement systems have been developed to enhance the efficiency of the drive system, to reduce the risk of cam-out as well as to improve other aspects of the drive system. One such system that has had use in aircraft applications is described in U.S. Pat. No. Re. 24,878 (Smith et al.). The recesses are defined by three or four wings that extend radially from the central portion of the recess. Each wing defines an installation wall and a removal wall, both of which are designed to be substantially vertical, that is, to lie in a plane that parallels the central axis of the fastener. The driver has a complementary configuration. The system is intended to promote axial alignment of the driver and fastener by eliminating the tendency for the driver to xe2x80x9crockxe2x80x9d in the recess as well as to resist cam-out. Axial alignment of and full seating of the driver within the recess is essential to obtain the benefit of the vertically oriented driving and removal walls. Even with this system, application of high torque loads may cause some deformation of the wings of the driver in a manner that tends to reduce the area of the region of contact between the driver blades and the walls of the recess wings as well as to shift the contact region radially inwardly. That, in turn, increases the risk of plastic distortion of the recess that can lead to progressive deterioration of the driver-recess engagement with resulting adverse consequences.
Another driver-recess engagement system is described in U.S. Pat. No. 3,237,506 (Muenchinger) that has been incorporated in fasteners commercially developed for commercial application under the trade designation Pozidriv(copyright). Among the characteristics of the Pozidriv(copyright) system is that the sidewalls of each of the wings of the recess is designed to lie in a plane that is substantially vertical. In forming such substantially vertical sidewalls by the conventional cold header technique in which a two-blow heading machine impacts the end of the wire or other material from which the fastener is made, while the wire is supported in a die of the heading machine, first with a punch that forms a bloom (a partially formed head) on the end of the fastener blank and then with a finishing punch that finishes the head and forms the driver-engageable recess. The operation is carried out automatically and at high speed. The punches are impacted against and withdrawn from the head end of the fastener blank along the longitudinal axis of the fastener blank. Among the constraints inherent in the heading process is that the recess design should be free of undercut regions, that is, regions that, although being formable as the punch is impacted into the fastener head, would be obliterated when the punch is retracted. The Muenchinger patent describes a recess and punch configuration intended to reduce or eliminate the effect of xe2x80x9cmetal fall-awayxe2x80x9d that tends to occur when punching a recess in a fastener head. The result of the phenomenon of metal fall-away is that the recess does not conform precisely to the configuration of the recess-forming punch. The lack of accurate conformance results in a recess that will exhibit increased cam-out and will reduce driver stability, resulting in a loose, wobbly fit between the driver and the recess. Those difficulties result in a reduced torque capacity of the mated fastener and driver.
U.S. Pat. Nos. 4,187,892 (Simmons) and 5,120,173 (Grady) describe a drive system for threaded fasteners in which intentionally deformable ribs are provided on two or more of the driver-engageable sidewalls of one or both of the recess and driver. The ribs project slightly from the sidewalls and are designed to deform or cause deformation of the ribs they engage in a manner that provides an interlocking engagement. The interlocking engagement of the ribs resists cam-out. Such anti-cam-out ribs may be incorporated in recesses having substantially vertical drive walls as well as those recesses in which the sidewalls are inclined substantially from the vertical. The ribs are formed during the cold heading process in which the recessed fastener head is formed in a two-blow heading machine.
Although the inclusion of anti-cam-out ribs on the recessed head fasteners significantly improves the drive performance of the fasteners, some types of recessed head fasteners may require compromises in the design in order to include the advantages of the anti-cam-out ribs. The anti-cam-out ribs cannot be formed in a manner that would leave an undercut that would result in the rib being torn out as the recess forming punch is retracted. In recesses with substantially vertical sidewalls the anti-cam-out rib also must be essentially vertical and parallel to the fastener axis. Although it would be desirable to locate such ribs at a maximum radial distance from the central axis of the fastener, that is, at the more radially outward regions of the recess sidewalls, the height of the recess sidewalls progressively decrease in height toward their radial extremities. Consequently, the vertical height of the rib necessarily is very short toward the radial extremities. Such a rib can only be engaged by a ribbed driver blade near the upper end of the recess adjacent the top surface of the fastener head. A short rib, so located, presents greater risk of improper engagement or non-engagement by the ribbed driver. Consequently, an anti-cam-out rib, particularly in a recess having vertical or near vertical drive walls, typically has been located more radially inwardly along the sidewall.
Also among the compromises to be considered when employing a ribbed recess is that the protrusion of the rib into the envelope of the recess wing necessarily requires either that the width of the wing be increased or that the width of the mating driver blade be reduced to accommodate that projection. That, in turn, requires a reduction in the mass of material of the fastener head, or the driver blades, or both, in order that the driver blade can be properly inserted into the recess. This design compromise may be particularly pronounced in those applications where it is desirable to provide an anti-cam-out rib on both the installation and the removal sidewalls of the recess wings. Additional complications result from the desirability that such recesses should be compatible with existing drivers. In many cases, the inclusion of anti-cam-out ribs in the recess may restrict the extent to which an existing driver can penetrate into the recess, possibly preventing full depth penetration, consequently reducing the effectiveness of torque transmission.
The anti-cam-out ribs that have been incorporated into recessed head fastener systems typically have had a V-shaped cross-sectional profile that defines a relatively sharp apex along the length of the rib. When such a fastener is intended for use in an environment where a plating will enhance its function (e.g., an anti-corrosive plating), the very small area of intersection of the driver and recess ribs can be expected to result in high stresses sufficient to fracture the plating, particularly when the driver also has ribs on its blades.
It would be desirable to provide improvements in recessed head fasteners and drivers by which the foregoing and other difficulties are reduced or eliminated.
The present invention is adapted for use in a fastener recess having a central portion and a plurality of wings radiating from the central portion. The wings are defined by a pair of sidewalls and an end wall. Two or more of the sidewalls of the wings includes a relieved region that extends from the upper edge of the sidewall downwardly toward the bottom of the sidewall. The width of the relieved region, measured radially along the sidewall, may decrease progressively from the top to the bottom of the sidewall. The relieved region is configured to leave an elongate strip referred to herein as a xe2x80x9ctorque padxe2x80x9d extending along the outer margin of the sidewall. The torque pad is adapted to be engaged by the radially outer margin of the driver blade, while the relieved region remains spaced slightly from the driver blade. When the driver is mated with the recess, the outer margins of the driver blades will bear against the torque pads, while the more radially inwardly disposed portions of the blades will remain spaced slightly from the sidewall in the relieved region, unable to transmit force to the relieved region. The torque developed by the driver thus will be applied to the screw head along the torque pads, thereby assuring that the driving force will be applied at the most radially outward regions of the recess sidewalls. By so maximizing the moment arm of the force applied by the driver blades, maximum torque can be transmitted to the screw without applying excessive forces to the recess sidewall. Consequently, the risk of adverse plastic deformation of the recess is reduced.
In another aspect of the invention, the recess can be formed to leave an additional unrelieved rib extending along the inner margin of the drive walls. Such inner rib may serve to engage the innermost regions of the driver blade, near the root of the blade, to provide additional stabilization for the mated driver. By providing both the torque pads at the outer margin of the wing sidewalls and stabilizing ribs at the inner margin the risk of the driver misalignment with the recess is reduced.
Another aspect of the invention relates to the configuration of the heading punch by which the recess may be formed. The punch is provided with raised surfaces on one or both of the drive walls of the punch wings, the raised surfaces being adapted to form the relieved regions in the sidewall of the corresponding recess. None of the surfaces that defines a raised region is oriented in a manner to form undercut surfaces in the resulting recess. All of the surfaces of the raised region are oriented to provide a positive draft angle. Consequently, when the punch forms the recess, it can be withdrawn without damaging or tearing out any of the formed surfaces. The geometry of the raised region can be selected in a manner to define the geometry and dimensions of the resulting torque pad and stabilizing rib of the resultant recess to provide desired recess characteristics. The configuration of the punch is such that it permits the formation of a torque pad in a substantially vertical or non-vertical recess sidewall in which the torque pad is inclined at a substantial angle to the longitudinal axis of the fastener.
In a further aspect of the invention, a mating driver is provided and includes one or more ribs projecting from the sidewalls of the driver. The width of the recess wing is defined between the unrelieved surfaces of the drive walls, may be considered to define a recess envelope receptive to the envelope of a driver bit, the envelopes being dimensioned to mate closely to each other within the limits of practical tolerances. The envelope defined by a driver, including the driver ribs, is selected to correspond to the envelope defined by an unrelieved recess. This enables the driver to be mated properly with the recess, whether the recess is a conventional commercially available recess or is modified in accordance with other aspects of the present invention, summarized above. The elongate driver ribs are oriented to extend generally perpendicular to the outer wing wall of the driver blade so that when mated with a recess formed in accordance with the invention, the driver ribs will extend generally perpendicular to the orientation of their associated torque pad. The ribs can engage and dig into the torque pad sufficiently to enhance the resistance to camout while doing so with substantially less deformation than results from engagement of driver ribs with a conventional anti-camout rib on the wing wall. In a further aspect of the invention, the ribs of the driver are formed to taper in a radially inward direction, thereby enabling the transverse cross-section of the driver ribs to increase in a radially outward direction but without increasing the dimensions of the driver envelope beyond those of the recess envelope.
It is among the objects of the invention to provide an improved drive system for a recessed head threaded fastener.
Another object of the invention is to provide a recessed head threaded fastener in which the driving force by a mating driver is applied to the radially outer regions of the drive wall(s) of the recess to maximize the moment arm of the driving forces applied to the fastener.
Another object of the inventions is to provide an improved recessed head threaded fastener in which a selected level of torque may be developed while applying a reduced level of stress to the fastener, as compared with the stress that would otherwise be applied in the absence of the invention.
A further object of the invention is to provide an improved ribbed driver adapted for use with a recess made in accordance with the invention yet which also is compatible with conventional recesses.
Another object of the invention is directed to a recessed head fastener having a recess of the type described in which the recess is compatible with conventional drivers as well as the improved driver of the present invention.
A further object of the invention is to provide a driver having ribs located and oriented to effect maximum torque transmission while reducing the damage to the recess.
Another object of the invention is to provide a driver for a recessed head fastener that includes ribs oriented generally perpendicular to the end wall of the blades of the driver.
Another object of the invention is to provide a driver of the type described in which the ribs on the driver blades are adapted to engage a torque pad on a recess formed in accordance with the invention.
Another object of the invention is to provide a recess for a threaded fastener in which the applied force of the driver to the recess is in the radially outermost regions of the wings of the recess and in which a stabilizing rib is provided at the radially inwardly regions of the wings to stabilize the driver with respect to the recess.
Another object of the invention is to provide a header punch for fabricating recessed head fasteners in accordance with the invention.
A further object of the invention is to provide a method for fabricating such recessed head fasteners.